A colour display tube comprises an in-line electron gun for generating three co-planar electron beams having a mean lens comprising a first and second lens electrode for producing a focusing lens field for focusing the electron beams. At least one of said lens electrodes has a correction element for tuning the astigmatism of the lens field. The correction element comprises three co-linear apertures for passing the electron beams, the outer apertures of which are funnel-shaped, the wide end of the funnel-shape being directed away from the centre aperture.